Lymphoma cell variants of decreased glucocorticoid sensitivity

Glucocorticoids repress bcl-X expression in lymphoid cells by recruiting STAT5B to the P4 promoter

The bcl-X gene plays a critical role in apoptosis. Six different isoforms generated by tissue-specific promoter usage and alternative splicing were described. Some of them exert opposite effects on cell death. In mammary epithelial cells glucocorticoids induce bcl-X expression and increase the ratio bcl-X(L) (antiapoptotic)/bcl-X(S) (apoptotic) by activating P4 promoter, which contains two hormone response elements. Here we show that, on mouse thymocytes and T lymphocyte derivative S49 cells, glucocorticoids inhibited transcription from P4 and decreased the ratio bcl-X(L)/bcl-X(S) favoring apoptosis. Upon hormonal treatment, glucocorticoid receptor (GR), steroid receptor coactivator-1, and RNA polymerase II were transiently recruited to P4 promoter, whereas STAT5B was also recruited but remained bound. Concomitant with the release of GR, silencing mediator for retinoic acid receptor and thyroid hormone receptor and histone deacetylase 3 were recruited, histone H3 was deacetylated, and RNA polymerase II left the promoter. Inhibition of STAT5 activity reverted glucocorticoid repression to activation of transcription and was accompanied by stable recruitment of GR and RNA polymerase II to P4.

[Clinical relevance of glucocorticoid receptors in the treatment of lymphoid neoplasias]

A generalized view on the mechanism of steroid hormone action is presented with special emphasis on glucocorticoids and their lymphocytolytic effects. The present knowledge on the structure and function of glucocorticoid receptors is reviewed. Following hormone binding, the receptor complex is activated to a form which is able to interact with chromatin or DNA. Several types of receptor mutants have been obtained in an animal cell culture system which allows selection of cell variants. The biochemical analysis of these mutants helped to establish a domain model of receptor structure. The quantitative effect of receptor numbers and hormone activity on lymphocytolysis is described in a cell culture model system and the results are discussed in view of the rational treatment of lymphoid neoplasia with glucocorticoids. The clinical experience with glucocorticoids alone and in combination therapy with cytostatic drugs is summarized. Our special emphasis is on acute lymphoblastic leukemia and malignant lymphoma as patients with these diseases can now be treated with remarkable success rates.

Genetics of glucocorticoid receptors

The lymphocytolytic effect of glucocorticoids has been used for isolating receptor mutants. They fall into several groups with defects either in the hormone binding domain or the DNA binding domain or with part of the receptor polypeptide missing. These truncated receptors have increased binding affinity for general DNA and are synthesized from 5'-truncated messages. In addition, a phenotype has been identified in which a receptor allele, although apparently normal, is shut-off with no gene product detectable. The wild-type receptor polypeptide of about 95,000 molecular weight is synthesized from two mRNAs of 7 kb and 5 kb which differ in the lengths of their 3'-untranslated regions. A receptor model with three linearly arranged and functionally distinct domains is discussed. The DNA binding domain is rich in basic amino acids and cysteines and is located in the middle of the polypeptide. This region has the highest degree of homology with the estrogen receptor and with the v-erb-A oncogene product.

Down-regulation of glucocorticoid receptor mRNA by glucocorticoid hormones and recognition by the receptor of a specific binding sequence within a receptor cDNA clone

A cDNA clone for the rat glucocorticoid receptor (GR) was used to study mechanisms of GR mRNA regulation. Treatment of rat hepatoma culture cells with 0.5 microM dexamethasone caused a small, initial increase in the GR mRNA level after 6 hr as well as a 50% to 95% reduction of the GR mRNA level after 24 hr of incubation when studied by RNA blot hybridization. After 72 hr, the initial GR mRNA level was restored. The down-regulation of GR mRNA levels appears to be independent of protein synthesis, since it also was observed in the presence of cycloheximide. However, cycloheximide caused a 4-fold increase in intracellular levels of GR mRNA. Using an immunoprecipitation assay, we could demonstrate that the GR specifically interacts with a GR cDNA clone, which represents a 2.6-kilobase fragment of the 3' nontranslated region of the GR mRNA. Nuclease protection experiments indicate the presence of several internal GR-binding regions in the above fragment.

Immunochemical characterization of wild-type and variant glucocorticoid receptors by monoclonal antibodies

Monoclonal antibodies raised against the rat liver glucocorticoid receptor were used to investigate receptors of wild-type and glucocorticoid-resistant variants of mouse lymphoma cells. Two of the variant types contained receptors of 'nuclear transfer deficient' (nt-) and 'increased nuclear transfer' (nti) phenotypes, respectively, while the third was of the 'receptorless' (r-) phenotype with negligible hormone binding activity. Three monoclonal antibodies of the IgM class and one of the IgG class reacted with both wild-type and nt- receptors but not with the steroid binding form of nti receptors. Some of the antibodies bound the wild-type and nt- receptors more efficiently after activation at 20 degrees C. By use of an immuno-competition assay we were able to detect cross-reacting material in considerable amounts in extracts of nti and r- cell variants. This material was further characterized by gel filtration and immunoblotting. The immunoreactive material of wild-type, nti and r- cells gave a major band of mol. wt. 94 000 upon SDS-gel electrophoresis while the steroid-binding polypeptides of wild-type and nti receptors have mol. wts. of 94 000 and 40 000, respectively. The data show that in S49.1 mouse lymphoma cells the products of two receptor alleles can be distinguished.

Cellular receptor levels and glucocorticoid responsiveness of lymphoma cells

A series of mouse lymphoma cell lines of independent origin was investigated with respect to glucocorticoid sensitivity, cellular receptor levels, and properties of receptors. The concentrations of the glucocorticoid dexamethasone required to produce comparable growth-inhibitory effects varied considerably amongst these cell lines. Also a wide range in the number of receptors per cell was found. When the receptor-steroid complexes were compared with respect to nuclear binding properties and affinities for DNA, no differences were seen. For 7 out of 10 cell lines studied we obtained a direct correlation between hormonal sensitivity and the number of cellular receptor sites divided by the dissociation constants KD for the receptor-dexamethasone complexes. This suggests that the receptor is a major quantitative determinant for steroid responsiveness. The limitations of receptor measurements for glucocorticoid therapy of lymphoid neoplastic disease are discussed.

Glucocorticoid receptor number and intracellular water space

In order to elucidate the relationship between cell water content and number of glucocorticoid receptors, eleven normal and malignant lymphoid or myelomonocytic cell types originating from mouse, rat and man were investigated. The cellular water space was measured with 3H2O, and glucocorticoid receptor number was measured in a whole-cell binding assay with [3H]dexamethasone at 30 and 37 degrees C. The intracellular water phase concentration of glucocorticoid receptors (around 40 nmol/l cell water), and the dependence of receptor affinity on temperature were similar in normal and malignant rodent and human cells. It is concluded that comparisons of glucocorticoid receptor levels are best made on the basis of intracellular receptor concentrations.